Injection-molded plastic container with improved stacking strength

ABSTRACT

A thermoplastic, injection-molded container provides stacking strength with reduced thickness walls by the addition of a series of shallow, spaced vertical buttresses integrally molded into the container sidewalls.

CROSS-REFERENCE TO RELATED APPLICATIONS

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STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

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BACKGROUND OF THE INVENTION

The present invention relates to injection-molded plastic containers and in particular to a container design and method of manufacture providing improved stacking strength with reduced material requirements.

Injection-molded plastic containers are used for packaging and shipping a wide variety of products. Such containers normally have a lid that may snap into place over the container opening to seal product within the container. With the exception of some smaller containers intended for paint, it is generally desirable that the containers, when empty, nest within each other to minimize space during the shipment and storage of the empty containers.

Containers for product packaging must be produced at a low cost consistent with the container being disposed of when the contents of the container have been consumed. One way to reduce the cost of a container is to reduce the thickness of the container walls and thus the amount of plastic needed to mold the container.

Reducing the thickness of the container walls is subject to two constraints. First, the sidewalls must be thick enough to support loads anticipated in the stacking of the filled containers on top of one other. Many containers comply with specific strength requirements that may be tested using methods like ASTM D4577 and are regulated to meet the requirements of standards such as ASTM D4504-94, both hereby incorporated by reference. For example, industrial one-gallon containers must be loaded with 1,500 pounds for 15 seconds without collapse or buckling or rupture of any portion of the container corresponding to at least three tiers of stacked and loaded containers.

Second, the sidewalls must be thick enough to allow practical injection molding. High injection pressures are necessary to force thermoplastic into thin wall sections of large molds, necessitating more expensive injection molding equipment. High pressure injection of thin wall sections may result in substantial distortion of the wall as it cools, causing unacceptable warping in the container, cosmetic blemishes, or surface irregularities that interfere with the printing of a label on the outside of the container

When using high-density polyethylene, for example, these requirements normally dictate that a five-gallon container have a nominal wall thickness of 0.090 inches producing an average total weight of approximately 975 grams.

BRIEF SUMMARY OF THE INVENTION

The present inventor has recognized that relatively shallow buttresses may be molded to the sidewalls of a plastic container to substantially increase the stacking strength of the container for a given average wall thickness. The buttresses can thereby maintain stacking strength while allowing a reduction of the material needed to manufacture the container. Further, the buttresses appear to improve the flow of plastic into the mold, reducing the amount of injection pressure required for molding of thin wall sections reducing or avoiding distortion, and blemishes caused by high pressures.

Specifically then, the present invention provides a thermoplastic injection-molded container having a base and sidewalls and a lid releasably sealing to an upper edge of the sidewalls to enclose a container volume. The sidewalls have integrally molded variations in wall thickness forming a plurality of vertical buttresses.

It is one object of at least one embodiment of the invention to employ variable wall thickness to reduce the average wall thickness of an injection-molded container, thereby conserving thermoplastic material.

The difference in thickness of the buttresses over the thickness of the sidewalls away from the vertical buttresses is greater than two percent and less than thirty percent of the thickness of the sidewalls away from the vertical buttresses.

It is thus another object of at least one embodiment of the invention to use shallow buttresses that may be readily molded and that do not interfere with the use or stacking of the container. Shallow buttresses allow the molding of a container with a smooth outer wall free of shrinkage depression such as might be formed by larger rib structures, and thereby provide an improved appearance to the container and the ability to easily print directly on the outer surface of the container.

The vertical buttresses may extend inwardly from the inner surface of the sidewalls.

It is thus another object of at least one embodiment of the invention to provide improved contact between the buttress sections and the mold core, to which the container compresses during cooling, to shorten the cooling cycle of the injection-molding process.

The buttresses may extend substantially the entire height of the vertical sidewalls and may have a curved cross section to provide a continuous curved inner circumference to the container.

Thus it is another object of at least one embodiment of the invention to allow the mold core to be easily removed. By providing the buttresses with a shallow and smooth form, the molded container may be released from the mold core by simple flexure of the part eliminating the need for complex movable mold core portions.

The vertical buttresses may be separated by arcuate wall sections of substantially constant thickness.

Thus it is another object of at least one embodiment of the invention to permit the fabrication of the mold core by the simple addition of spaced grooves to the mold core.

The arcuate wall sections may have a width at least five times the width of the vertical buttresses times the difference in thickness of the vertical buttresses and the separating arcuate wall sections.

It is thus another object of at least one embodiment of the invention to substantially decrease the total material needed for fabricating the container by concentrating increased thickness buttress sections between larger areas of a reduced thickness wall.

The vertical buttresses may be separated by varying distances such that at least two vertical buttresses are adjacent to each other within a first distance and separated from at least two other vertical buttresses by a second distance greater than the first distance.

It is thus an object of at least one embodiment of the invention to allow multiple small buttresses to be placed adjacent to each other to obtain the benefit in strength of a larger buttress without increasing a concentration of plastic material such as may promote shrinkage or distortion in the container.

The sidewalls may taper outward toward the top of the container allowing the containers to be stacked in nested configurations and the thickness of the buttresses in excess of thickness of the sidewalls away from the vertical buttresses may be less than the taper of the sidewalls from the bottom to the top of the vertical buttresses.

It is thus another object of at least one embodiment of the invention to provide buttresses that do not jeopardize the ability of the container to be stacked.

The thickness of the buttresses in excess of the thickness of the sidewalls may be substantially constant along their vertical extent.

It is thus another object of at least one embodiment of the invention to provide a substantially constant width buttress independent of curvature of the sidewalls such as will resist bucking under loads.

The container may have a volume substantially equal to five-gallons, and the wall thickness away from the buttresses, may be less than eighty-five thousandths of an inch.

It is thus another object of at least one embodiment of the invention to substantially decrease the amount of material required for a standard five-gallon container.

The top end of the buttresses may be tapered into the sidewalls.

It is thus another object of at least one embodiment of the invention to provide a design that allows easy ejection of the mold core without movable mold core elements.

The molded container may further include horizontal buttresses extending across the base, for example, in a radial or concentric circular pattern.

It is thus another object of at least one embodiment of the invention to provide improved strength to the base of the container for supporting product in the container.

The buttresses may extend along the direction of plastic flow during the injection molding of the part.

It is thus another object of at least one embodiment of the invention to provide improved injection molding at lower injection pressures.

These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a five-gallon container such as may be produced using the present invention;

FIG. 2 is a fragmentary cross section taken along line 2-2 of FIG. 1 showing the variations in wall thickness such as provide pairs of adjacent buttresses separated by arcuate wall sections of constant thickness;

FIG. 3 is an enlarged fragmentary view of the buttresses of FIG. 2 showing relative dimensions and spacings of the buttresses;

FIG. 4 is a fragmentary cross section along line 4-4 of FIG. 1 showing a taper of the container wall in comparison to the thickness of the buttresses, the latter which does not interfere with a nesting of the container;

FIG. 5 is a top plan view of the base of the container of FIG. 1 showing a first embodiment providing an extension of the buttresses along the base to provide for additional support;

FIG. 6 is a figure similar to FIG. 5 showing a second embodiment providing an extension of the buttresses in concentric rings;

FIG. 7 is a figure similar to FIG. 2 showing an alternative embodiment in which buttresses are provided with substantially even spacing over the inside of the container;

FIG. 8 is a figure similar to that of FIGS. 2 and 7 showing an embodiment in which the buttresses are placed on the outside of the container; and

FIG. 9 is a simplified perspective view of a mold core for use in the present invention during molding of the container showing the alignment of the buttresses with the flow of thermoplastic from the injector such as improves the molding process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a container 10 of the present invention may provide a generally planar base 12 forming the bottom of the container 10. Attached at the periphery of the base 12 are upwardly extending sidewalls 18. The sidewalls 18 terminate at an upper rim 20 and may be sealed by attachment of a lid 24 to the upper rim 20 of the sidewall 18 to enclose a container volume 22. Each of the base 12, sidewalls 18 and lid 24 may be injection molded of a thermoplastic material such as a high density polyethylene, or polypropylene, or copolymers although other polyolefins may be used.

An outer surface 19 of the sidewalls has a smooth circular cross-section that may be contact printed, for example, using transfer or silkscreen printing with information 21 for the consumer. Rims and flanges 23 may be formed in the outer surface 19 to provide attachment points for the lid 24 and supports for handles (not shown) and the like.

When filled with a product (not shown) and sealed with the lid 24, the container 10 may be stacked along a vertical central axis 26 with other containers 10 (not shown), the weight of upper containers 10 being transmitted through the lids 24 into the sidewalls 18 and to the base 12 of lower containers 10.

Referring now to FIGS. 2 and 3 per the present invention, the sidewalls 18 of the container 10 may have a varying thickness measured generally along a line perpendicular to axis 26. This varying thickness produces buttress sets 28, being regions of relatively greater thickness, separated by arcuate wall sections 31 of relatively lesser constant thickness. Each buttress set 28 is separated by a buttress set separation distance 34 being also the width of the arcuate wall section 31. Generally, as will be described below, the buttress sets 28 provide a buttress thickness 36 that increases the thickness of the sidewall 18 and thus the amount of thermoplastic used in the container 10, but this increased thickness is more than offset by a reduction of wall thickness 38 of the constant thickness arcuate sections 31.

In a preferred embodiment, each buttress set 28 is formed of two adjacent, vertically extending buttresses 30 separated by a buttress pairing distance 32 to provide an “m” shaped cross section. The use of two adjacent buttresses 30 decreases a concentration of thermoplastic material such as may create sink distortions in the outer surface 19 of the container 10.

The difference between the thickness 36 at the buttresses 30 and the thickness 38 at the constant thickness arcuate sections 31 provides a buttress freestanding thickness 40. In one embodiment, the buttress freestanding thickness 40 is limited to less than the wall thickness 38. More generally the buttress freestanding thickness 40 may be greater than five percent and typically greater than 30 percent of the wall thickness, although these limits are simply acceptable ranges meeting the requirements of the invention and not limitations to the invention.

By way of example, the inventor has determined that, buttress sets 28 with a freestanding thickness 40 of only ten thousandths of an inch can allow the thickness 38 of the arcuate wall sections 31 to be reduced sufficiently to reduce the total material weight of a standard five-gallon container by eighteen percent. For such a container, the wall thickness 38 may be reduced to less than 85 thousandths of an inch, for example, to 80 thousandths of an inch as opposed to the standard 90 thousands of an inch wall thickness in a standard container to provide a reduced weight of 785 grams.

This savings in material provided by the buttress sets 28 can be determined by calculating the average wall thickness 35 being generally the area weighted sum of the thicknesses 36 and 38 over the entire sidewalls 18. The difference between the uniform wall thickness of a standard container and the average wall thickness 35 of container 10 times the area of the sidewalls 18 provides the material savings in the sidewalls 18.

In order to achieve this savings, the thickness 38 must be less than the uniform wall thickness of a standard container and the buttress set separation distance 34 must be sufficient to offset the material added to the sidewalls 18 by the buttresses 30. In one embodiment, for example, the ratio between the width 42 of the buttresses 30 times their freestanding thickness 40 compared to the separation of the buttresses 30 is greater than five to one. In the preferred embodiment, the width 42 of the buttresses is greater than their freestanding thickness 40.

Referring still to FIG. 3, the buttresses 30 have a curved inner surface in cross section to improve the release of the container 10 from a mold core 50 (shown in FIG. 18) to be described below. The inner surface of the buttresses 30 also provides increased conductive surface area between the mold core 50 and the buttresses 30, augmented by shrinkage of the container 10 in constriction about the mold core 50 as the thermoplastic material of the container 10 cools. This heat conduction path reduces the cooling cycle of the injection molding process.

Referring to FIG. 4, the buttresses 30 will extend the substantial height of the sidewall 18 and may include a tapered end 47 allowing the container 10 to be removed from a mold core (not shown) by simple flexure of the container itself while allowing the buttresses 30 to stop before the rim 20 to provide a smooth and unbroken area just inside the rim 20.

As shown in FIG. 4, the sidewall 18 may slope outward toward the rim 20 allowing the container 10 to nest with other similar containers 10 for storage or shipment. The buttress freestanding thickness 40 is substantially less than the taper distance 44, the latter being the difference in inside diameter at the rim 20 and at the base 12. In this way, the buttresses 30 can be ensured not to interfere with nesting of the containers 10.

Referring now to FIG. 5, the base 12 may include upstanding ribs 46 providing not stacking strength, per se, as with buttresses 30, but bending strength to prevent bowing out of the base 12 under the weight of product within the container 10. The ribs 46 may extend from the periphery of the base 12 in a radial pattern toward a center of the base 12, for example, and be essentially continuations of the buttresses 30 or, as shown in FIG. 6, may be a series of concentric circles providing the stiffening function.

Referring now to FIG. 7, in an alternative embodiment, the buttresses 30 may be regularly spaced about the inner surface of the sidewall 18 and still provide an average wall thickness 35 under that of a standard container with uniform wall thickness.

As shown in FIG. 8 in an alternative embodiment, the buttresses 30 may be placed on the outside of the wall when an inner smooth surface is required.

Referring now to FIG. 9, a mold core 50 may be used to mold the inner surface of the container 10 and may include a generally cylindrical core for a paint can or a tapered core, being a frustum of a cone, for nesting container 10. In either case, the container 10 may be removed from the mold core 50 by simple upward motion along the axis 26 with flexure of the material of the container 10 allowing the buttresses 30 to be released from the mold core 50 as a result of their smooth contour and shallow freestanding thickness 40.

Injection of thermoplastic from an injector 52 may be conducted from a point 54 centered on the base 12 of the container 10 so that flow of thermoplastic material 55 proceeds radially outward along base 12 and downward along the sidewalls in a path generally parallel to the length of the grooves 56 corresponding to the molding of the buttresses 30 (not shown in FIG. 9). The grooves 56 molding the buttresses 30 provide passageways for the rapid flow of thermoplastic in the mold reducing or eliminating the need for higher pressure to mold the thinner constant thickness arcuate sections 31, and preventing incomplete molding of the container or high stress of the thermoplastic in the constant thickness arcuate sections 31 such as could produce distortion or sink spots in the outer surface of the container. Alternatively the improved flow of thermoplastic provides for faster filling at a given pressure.

The present invention may also be used in non-cylindrical containers, for example, square containers and the like. The invention is applicable to a wide variety of container sizes including 1-12 gallon industrial round and square containers and fractional gallon containers including quart containers and the like.

It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. 

1. A thermoplastic, injection molded container comprising: a base; sidewalls extending upward from the base; a lid releasably sealing to an upper edge of the sidewalls to enclose a container volume; and wherein the sidewalls have integrally molded variations in wall thickness forming a plurality of vertical buttresses providing resistance to stacking loads.
 2. The thermoplastic, injection molded container of claim 1 wherein the vertical buttresses extend inward from an inside of the container.
 3. The thermoplastic, injection molded container of claim 1 wherein the vertical buttresses are separated by arcuate wall sections of substantially constant thickness.
 4. The thermoplastic, injection molded container of claim 3 wherein the arcuate wall sections have a width at least five times a width of the vertical buttresses.
 5. The thermoplastic, injection molded container of claim 1 wherein the vertical buttresses have a cross section providing a continuous curved inner circumference to the container.
 6. The thermoplastic, injection molded container of claim 1 wherein the vertical buttresses have a first thickness greater than a second thickness of the sidewalls away from the vertical buttresses, wherein the first thickness is selected to provide a substantially smooth and circular outer circumference of the container without depressions caused by shrinkage.
 7. The thermoplastic, injection molded container of claim 1 wherein the vertical buttresses are separated by varying distances such that at least two vertical buttresses are adjacent to each other within a first distance and separated from at least two other vertical buttresses by a second distance greater than the first distance.
 8. The thermoplastic, injection molded container of claim 1 wherein the sidewalls taper outward toward a top of the container allowing the container to be stacked in nested configurations, and wherein the thickness of the buttresses in excess of the thickness of the sidewalls away from the vertical buttresses is less than a taper of the sidewall from a bottom to a top of the vertical buttresses, whereby the buttresses do not interfere with stacking.
 9. The thermoplastic, injection molded container of claim 8 wherein the thickness of the buttresses in excess of the thickness of the sidewalls away from the buttresses is substantially constant along a vertical extent of the buttress.
 10. The thermoplastic, injection molded container of claim 1 wherein the thickness of the buttresses in excess of the thickness of the sidewalls away from the vertical buttresses is greater than 2% and less that 30% of the thickness of the sidewalls away from the vertical buttresses.
 11. The thermoplastic, injection molded container of claim 1 wherein the container volume is substantially five gallons and wherein wall thickness away from the buttresses is less than 85 thousands of an inch.
 12. The thermoplastic, injection molded container of claim 1 wherein the buttresses extend less than an entire height of the sidewalls.
 13. The thermoplastic, injection molded container of claim 1 wherein a top end of the buttresses is tapered into the sidewalls.
 14. The thermoplastic, injection-molded container of claim 1 wherein the container meets standard ASTM D4504-94 for stacking strength.
 15. The thermoplastic, injection molded container of claim 1 further including horizontal buttresses extending across the base.
 16. The thermoplastic, injection molded container of claim 15 wherein the horizontal buttresses extend radially from a center of the base.
 17. The thermoplastic, injection molded container of claim 15 wherein the horizontal buttresses extend circumferentially in a set of concentric loops about a center of the base.
 18. The thermoplastic, injection-molded container of claim 1 wherein the base is substantially circular and the upward extending sidewalls form a cylinder.
 19. A method of fabricating a container comprising: (1) molding a base and integral sidewalls extending upward from the base to define a container volume wherein the sidewalls have integrally molded variations in wall thickness forming a plurality of vertical buttresses; and (2) contact printing an outer surface of the container.
 20. A mold for a thermoplastic container comprising: a core body defining a container volume opening along a container axis and having therein a series of constant depth shallow grooves parallel to the container axis and extending substantially a height of the core body; a mold shell fitting about the core body to define therebetween a container having a base and sidewalls extending upward from the base to surround the container volume; and whereby the depth of the shallow grooves is such as to allow removal of a formed container from the core body by flexure of the formed container.
 21. The mold of claim 20 wherein the shallow grooves run parallel to a dominant direction of flow of thermoplastic into a mold formed by the core body and the mold shell and wherein the grooves run less than an entire length of the mold core. 